The overall goal of this study is to determine the cerebral activity patterns, as assessed by Functional Magnetic Resonance Imaging (FMRI), in human volunteers during the withdrawal from chronic nicotine self-administration by cigarette smoking and the ability of several cognitive tasks to activate the brain as a function of time of withdrawal and drug craving. FMRI has not been previously applied to demonstrate the consequences of any abused substance on cerebral activity in tolerant substance abusers.